1. Field of the Invention
The present invention relates to a light receiving device which selectively receives light having a specific wavelength range. More particularly, the present invention relates to a light receiving device which selectively receives a signal light beam having a longer wavelength among a plurality of signal light beams having different wavelengths.
2. Description of the Related Art
Currently, pin photodiodes made of a compound semiconductor are widely used as light receiving devices for the optical fiber communication. The pin photodiodes include a window structure for the purpose of improving the light reception sensitivity. In the pin photodiodes, a light absorbing layer having a narrow forbidden bandwidth (long absorption edge wavelength) is provided near a semiconductor substrate. A window layer having a wide forbidden bandwidth (short absorption edge wavelength) is provided on the light absorbing layer so that light having a wavelength between the absorption edge wavelengths is efficiently absorbed in the light absorbing layer. The absorption edge wavelength of a layer herein means the maximum wavelength of light absorbed by the layer. A most typical pin photodiode made of InGaAs/InP includes a light absorbing layer made of InGaAs and a window layer made of InP. In this case, assuming that this structure receives light from the window layer, the light absorbing layer can receive light in the absorption edge range between 0.9 xcexcm of InP and 1.65 xcexcm of InGaAs.
Pass-band pin photodiodes having sensitivity only to light in a narrower wavelength range have been developed. For example, when a wavelength multiplex communication is performed using a signal light beam having a wavelength of 1.3 xcexcm and a signal light beam having a wavelength of 1.55 xcexcm, a pass-band photodiode having sensitivity only to each wavelength is used. One of the pass-band photodiodes needs to have a pass-band characteristic in which a sensitivity to a signal light beam having a wavelength of 1.3 xcexcm is sufficient, but a sensitivity to the signal light beam having a wavelength of 1.55 xcexcm is substantially zero. Such a characteristic can be achieved if the light absorbing layer is made of InGaAsP having an absorption edge wavelength of 1.4 xcexcm, instead of InGaAs. With this configuration, the sensitivity to the wavelength 1.3 xcexcm can be separated 30 dB or more away from the sensitivity to the wavelength 1.55 xcexcm. This is because an electron-hole pair is not generated in the light absorbing layer by the light beam having a wavelength of 1.55 xcexcm. Although some absorption of light having a wavelength of 1.55 xcexcm occurs due to impurity levels in the forbidden band, such absorption has an extremely low efficiency. Therefore, substantially no photoelectric current occurs.
The other pass-band photodiode needs to have a pass-band characteristic in which there is a sufficient sensitivity to the signal light beam having a wavelength of 1.55 xcexcm but substantially no sensitivity to the signal light beam having a wavelength of 1.3 xcexcm. An example of a structure achieving such a characteristic is disclosed in Japanese Publication for Opposition No. 1-48663 (1989). In this publication, a heterojunction phototransistor, but not a pin photodiode, is provided as a light receiving device. Referring to FIG. 6, a heterojunction phototransistor 500 includes a collector layer 502, a base layer 503, an emitter layer 504, and a wavelength filter 505, which are provided on an upper side of a semiconductor substrate 501. A collector electrode 506 is provided on a lower side of the semiconductor substrate 501. An emitter electrode 507 is provided on an upper side of the wavelength filter 505. The collector layer 502, the emitter layer 504, and the wavelength filter 505 are of the same conductivity type as that of the semiconductor substrate 501. The base layer 503 has the opposite conductivity type to that of the layers 502, 504, and 505. The emitter layer 504 has a forbidden bandwidth larger than that of the base layer 503. The forbidden bandwidth of the wavelength filter 505 is intermediate between the forbidden bandwidths of the base layer 503 and the emitter layer 504.
The heterojunction phototransistor 500 has a current amplifying function of a transistor as well as a light receiving function. In terms of the light receiving function, the base layer 503 functions as a light absorbing layer of a pin photodiode, and the emitter layer 504 functions as a window layer thereof. Unless the wavelength filter 505 is provided, the base layer 503 in the heterojunction phototransistor 500 has a high sensitivity to light in the absorption edge range from the absorption edge of the emitter layer 504 to the absorption edge of the base layer 503. Unfortunately, the wavelength filter 505 absorbs light having a wavelength corresponding to the absorption edge of the wavelength filter or less. Therefore, the heterojunction phototransistor 500 has a pass-band characteristic in which only light having a wavelength longer than the absorption edge of the wavelength filter 505. In order to achieve the selective light reception in which the wavelength of 1.3 xcexcm is rejected and the wavelength of 1.5 xcexcm is selected, for example, the absorption edge wavelength of the emitter layer 504 is set to 0.9 xcexcm, the absorption edge wavelength of the base layer 503 is set to 1.65 xcexcm, and the absorption edge wavelength of the wavelength filter 505 is set to 1.4 xcexcm. Such settings allow achievement of a long wavelength pass-band characteristic in which a sensitivity to a signal light beam having a wavelength of 1.55 xcexcm is high, but a sensitivity to the signal light beam having a wavelength of 1.3 xcexcm is low.
Japanese Laid-Open Publication No. 9-83010 discloses a pin photodiode which achieves a selective-wavelength capability using the above-described heterojunction phototransistor structure. This example has a complicated structure which includes two pin photodiodes so as to receive light having two wavelengths and further includes other electronic devices. Only the selective-wavelength capability will be described in the following example. Referring to FIG. 7, the heterojunction phototransistor 600 includes a wavelength filter 602, a buffer layer 603, a light absorbing layer 604, and a window layer 605, which are successively provided on a semiconductor substrate 601. An island-like diffusion region 606 in which p-type impurities are diffused is provided in the window layer 605. The light absorbing layer 604 under the diffusion region 606 functions as a light receiving region. A negative electrode 607 is provided on the diffusion region 606. A positive electrode 608 is deposited over a portion of the semiconductor substrate 601 which has been exposed by etching the window layer 605 and the light absorbing layer 604 (on the buffer layer 603). In the example, a signal light beam enters from below the semiconductor substrate 601. The absorption edge wavelengths of the light absorbing layer 604 and the wavelength filter 602 are set to 1.65 xcexcm and 1.4 xcexcm, respectively. Such settings allow achievement of a long wavelength pass-band characteristic in which a sensitivity to a signal light beam having a wavelength of 1.55 xcexcm is high, but a sensitivity to the signal light beam having a wavelength of 1.3 xcexcm is low.
Among the above-described conventional techniques, the heterojunction phototransistor 500 shown in FIG. 6 receives a signal light beam from the upper side thereof. The phototransistor 500 is in the shape of mesa which is created by etching a region which has been doped during crystal growth. Such a mesa-type light receiving device has a drawback in that a leakage current is likely to occur.
The heterojunction phototransistor 600 shown in FIG. 7 is of a planer type, having a window layer which is caused to be of the p-type layer by the diffusing impurities. Although the transistor 600 has a small leakage current, it receives a signal light beam from the lower side thereof. In the light receiving device which receives light from the lower side thereof, a tail current may occur when part of a signal light beam enters the light absorbing layer other than through the light receiving region. In general, incident light to the light receiving region (the light absorbing layer under the diffusion region) excites an electron-hole pair. The pair is split by an electric field into an electron and a hole, the electron reaching the semiconductor substrate and the hole reaching the diffusion region, thereby immediately generating a photoelectric current. When light enters a portion of the light absorbing layer other than through the light receiving region, an electron-hole pair is also generated. However, such portion of the light absorbing layer has substantially no electric field. Therefore, the hole migrates by diffusion. Thus, the hole migrates for a longer time before reaching the diffusion region. This causes a photoelectric current generated when light enters the portion of the light receiving layer to have a much slower response than that of the photoelectric current generated when light enters the light receiving region. The photoelectric current component having a much slower response is called tail current, which may be a significant problem in some applications of the light receiving device. In the light receiving device which receives light from the lower side thereof, the ring electrode, the conductor, the pad, and the like are provided on the upper side of the device, all the light receiving surface other than the diffusion region are not covered with a light shielding film. In the light receiving device which receives light from the upper side thereof, substantially the entire upper surface of the light receiving device other than the diffusion region can be covered with a light shielding film, thereby making it possible to significantly reduce generation of the tail current.
According to an aspect of the present invention, a light receiving device includes a semiconductor substrate; a light absorbing layer provided on the semiconductor substrate; a window layer provided on the light absorbing layer; a wavelength filter provided on the window layer; and a diffusion region provided in the wavelength filter and the window layer. A forbidden bandwidth of the wavelength filter is smaller than a forbidden bandwidth of the window layer; and a forbidden bandwidth of the light absorbing layer is smaller than the forbidden bandwidth of the wavelength filter.
According to another aspect of the present invention, a light receiving device includes a semiconductor substrate; and a light receiving region provided on the semiconductor substrate; and a wavelength filter provided in such a way as to cover the light receiving region.
In one embodiment of this invention, the light receiving device further includes a negative electrode provided on the light receiving region; and a pad connected to the negative electrode.
In one embodiment of this invention, the wavelength filter is an absorption filter including a compound semiconductor thin film.
In one embodiment of this invention, the wavelength filter is an interference filter including a dielectric multilayer film.
In one embodiment of this invention, the wavelength filter is provided with a resin.
In one embodiment of this invention, the wavelength filter is provided with a bump.
In one embodiment of this invention, wherein the bump is a solder bump.
In one embodiment of this invention, the light receiving device further includes a light absorbing layer functioning as a light receiving region; and a window layer provided on the light absorbing layer. A forbidden bandwidth of the wavelength filter is smaller than a forbidden bandwidth of the window layer; and a forbidden bandwidth of the light absorbing layer is smaller than the forbidden bandwidth of the wavelength filter.
According to another aspect of the present invention, a method for fabricating a light receiving device, includes the steps of forming a light receiving region on a first semiconductor substrate; forming a wavelength filter on a second semiconductor substrate; attaching the first semiconductor substrate to the second semiconductor substrate in such a way that the processed surfaces of the first semiconductor substrate abut with processed surfaces of the second semiconductor substrate; and exposing the wavelength filter by etching the second semiconductor substrate.
In one embodiment of this invention, the method further includes the step of etching the wavelength filter in such a way to cover the light receiving region.
In one embodiment of this invention, the attaching step includes the step of attaching the first semiconductor substrate to the second semiconductor substrate with a polyimide resin, and the method further includes the step of removing a portion of the polyimide resin not covered with the wavelength filter by dry etching after exposing the wavelength filter.
In one embodiment of this invention, the method further includes the step of forming an attaching pad on one of the first and second semiconductor substrates, and forming a bump on the other of the first and second semiconductor substrates, before the attaching step. The attaching step includes the step of fusion bonding the bump with the attaching pad.
According to another aspect of the present invention, a method for fabricating a light receiving device, includes the steps of forming a light receiving region on a first semiconductor substrate: forming a wavelength filter on a second semiconductor substrate; etching the wavelength filter, leaving a portion of the wavelength filter; attaching the first semiconductor substrate to the second semiconductor substrate in such a way that processed surfaces of the first semiconductor substrate abut with processed surfaces of the second semiconductor substrate; and exposing the wavelength filter by etching the second semiconductor substrate.
In one embodiment of this invention, the attaching step includes the step of attaching the first semiconductor substrate to the second semiconductor substrate with a polyimide resin, and the method further includes the step of removing a portion of the polyimide resin not covered with the wavelength filter by dry etching after exposing the wavelength filter.
In one embodiment of this invention, the method further includes the step of forming an attaching pad on one of the first and second semiconductor substrates, and forming a bump on the other of the first and second semiconductor substrates, before the attaching step. The attaching step includes the step of fusion bonding the bump with the attaching pad.
Thus, the invention described herein makes possible the advantages of (1) providing a planer-type light receiving device which has a long wavelength pass-band characteristic, in which a signal light beam having a longer wavelength is selectively received from among a plurality of signal light beams having different wavelengths, and which receives a signal light from the upper side thereof; and (2) providing the light receiving device further having a sufficiently large sensitivity ratio.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.